1. Field of Invention
The present invention relates to a thread comprising at least two strands of continuous carbon fiber twisted around one another, a process for the production of same, and the uses for this type of thread.
2. Description of Related Art
One of the things needed for the increased manufacture and utilization of textile preforms such as fiber composite materials and filter media, for example for industrial applications is suitable sewing thread. The purpose of these threads is to stabilize the textile preform, but they are also increasingly being used to structurally reinforce it, sometimes at very high temperatures, as are required, for example, for producing fiber ceramics or when using filter media in processes that are subject to high chemical and/or thermal stress.
The mechanical properties and the thermal and chemical stability of carbon fibers make this material particularly suitable for sewing thread, particularly for the aforementioned applications.
The many, mainly forceful thread diversions that occur during sewing damage the thread, and it usually does not achieve its original mechanical stability in the later composite. Brittle carbon fibers in particular can only be sewn under certain conditions. The theoretically attainable mechanical properties of the fibers are not achieved in the component after sewing.
Traditionally, a product made by Toray called Torayca T900 could be sewn only under certain circumstances. This thread is made from 1000 filaments or from two components with 1000 filaments each or three components with 1000 filaments each. The individual components have a twist in the range of S222-225 turns per meter (hereinafter “t/m”). If two or three components are twisted together into strands, these strands are twisted around one another at a twist of approximately Z162-Z164 t/m. Using a normal twisting process with many abrasive thread diversions, it is probably only possible to produce this twisted thread with thin, and thus pliable, filaments that have a diameter of around 5.5 μm or smaller. But it is very costly to produce carbon fibers with a diameter of less than 6 μm, so this type of thread is very expensive.
Alternatively, sewing threads have been developed that have a carbon fiber core and are additionally sheathed by another thread (cf., for example, JP-A 2133632 or JP-A 1061527. Various processes can be used to produce the sheath such as, for example, winding or crocheting around the core. Since the sheathing process is highly stressful for the thread material, polyester or polyamide threads, for example, are used. However, these threads have a low bond strength to the plastic matrix and reduce the volume of carbon fibers in the fiber composite material by the volume of the sheathing thread. Furthermore, the core materials of the sewing threads cannot be seated closely enough to the item being stitched, since the voluminous sheath is situated between them.
Alternatively, twisted glass threads, or aramid or PBO fibers, are used for sewing since they have a greater transverse strength than carbon fibers and so suffer less damage during the abrasive stitching process. But their compressive or mechanical properties in a composite with a matrix such as plastic are considerably more marginal than those of carbon fibers, so they cannot provide true structural reinforcement.
Usually the filaments of filament thread fibers are parallel in the thread after manufacture and do not form a closely integrated thread composite. However, for sewing threads in particular, such a composite is important since this is the only way to ensure that faultless seams are produced.
Twists are introduced to filament threads to produce a closely integrated thread composite. This, usually additional, production step represents an initial damaging of the filaments, which is in turn a source of further filament damage, to the point of the thread tearing, in the subsequent stitching process.